How do I add more than 8 decals to an object?
Image above.  Rendered image with two imagined logos on particular faces.


Image above.  UV Maps.  Color Blue used to increase visibility of Seams.

Mapping Extension set to Clip.
Even if you see Blender Render the UV Map applies to Cycles Render as well. Combined with the video tutorials you have seen you can do this. Note the part of UV is off the image which is what I want.  The Clip setting allows us to exclude modifications of color from parts of the mesh.  Please click to see larger image.

What is presented here is a quick summary, not a tutorial.